RFC 6376 — DomainKeys Identified Mail (DKIM) Signatures (allows emails to be signed by a particular domain to ensure they haven't been tampered with, and to say that that domain claims responsibility for the message).

Service discovery

Email software that wants to access a user's email account has to know the server(s) to connect to. This used to be manually configured, but nowadays is often done using the user's email address through a service discovery process.

Note that some software vendors seem to maintain their own database of email domain → server name definitions to support auto-configuration in their email clients. These seem to be custom databases maintained by each software vendor separately.

Filtering

RFC 5228 — Sieve: An Email Filtering Language (language used to file/filter/forward emails, what our Rules system ultimately generates. More information about sieve and sieve extensions. Note that our server doesn't support all the extensions.)